Nitrided tantalum columbium and vanadium-rich alloys

ABSTRACT

A NOVEL GROUP OF NITRIDED ALLOYS HAVING EXCELLENT WATER AND ABRASION RESISTANCE CONTAINING (A) ONE OR MORE METALS OF THE GROUP TANTALUM, COLUMBIUM, AND VANADIUM, (B) ONE OR MORE METALS OF THE GROUP TITANIUM AND ZIRCONIUM, AND (C) ONE OR MORE METALS OF THE GROUP MOLYBDENUM OR TUNGSTEN. THE ALLOYS CAN BE READILY FABRICATED TO SHAPE AND THEN HARDENED BY NITRIDING TO PRODUCE A CONTINOUS HIGH HARDNESS SURFACE LAYER SUPPORTED ON A GRADED NITRIDED LAYER.

United States Patent Office 3,674,572 Patented July 4, 1972 U.S. Cl. 148-315 2 Claims ABSTRACT OF THE DISCLOSURE A novel group of nitrided alloys having excellent wear and abrasion resistance containing (a) one or more metals of the group tantalum, columbium, and vanadium, (b) one or more metals of the group titanium and zircomum, and (c) one or more metals of the group molybdenum or tungsten. The alloys can be readily fabricated to shape and then hardened by nitriding to produce a continuous high hardness surface layer supported on a graded nitrided layer.

BACKGROUND OF THE INVENTION Our invention relates to a novel group of nitrided ternary or higher alloyed metals which alloys contain essentially at least one metal selected from each of the Groups I, II, and HI, wherein Group I consists of tantalum, columbium, and vanadium Group H consists of titanium and zirconium Group 111 consists of molybdenum and tungsten in amounts of percentage by weight as hereinafter set forth.

The nitriding of titanium-rich alloys, i.e., containing about 90% titanium has been studied previously (for example, see E. Mitchell and P. J. Brotherton, J. Institute of Metals, vol. 93 (1964), p. 381). Others have investigated the nitriding of hafnium-base alloys (F. Holtz, et al., US. Air Force Report IR-718-17 (H), 1967); molybdenum alloys (US. Pat. 3,161,949); and tungsten alloys (D. I. Iden and L. Himmel, Acta Met, vol. 17 (1969) p. 1483). The treatment of tantalum and certain unspecified tantalum base alloys with air or nitrogen or oxygen is disclosed in US. Pat. 2,170,844 and the nihiding of columbium is disclossed in the paper by R. P. Elliott and S. Komjathy, AJEME Metallurgical Society Conference, vol. 10, 1961, p. 367.

It is well known that elemental tantalum and columbium can be reacted with nitrogen to form hard continuous nitride surface layers. We have also found that the same can be accomplished with elemental vanadium. The nitriding kinetics for this element are more rapid and in general it is more desirable to nitride vanadium at lower temperature to suppress volatilization thereof. All three such elements form dispersed subnitride phases beneath the continuous surface layer. However, particularly in the case of columbium, the amount of such formation is restricted and little hardening occurs. As a result, there is a sharp transition from the hard continuous outer nitride layer to the relatively soft underlying material. This is an undesirable structure for many applications involving wear and abrasion since the hard outer layer is not well supported and can be chipped from the surface. We have discovered that by alloying with the combination of Groups I, II and III elements listed above, and subsequently nitriding, a finer nitride dispersion can be developed beneathv the continuous nitride layer which results in greater hardening to a considerable depth. Such materials have markedly superior erosion resistance compared to the nitrided elemental metals.

Accordingly, a principal object of our invention is to provide novel nitrided alloys containing (a) columbium, tantalum and/ or vanadium; (b) titanium and/or zirconium; and (c) molybdenum and/or tungsten, which alloys are quite high in columbium, tantalum and/ or 'vanadium and which nitrided alloys are characterized by a con tinuous nitride surface well supported by an underlying, graded nitride phase.

This, and other objects, features and advantages of our invention will become apparent to those skilled in this art from the following detailed disclosure thereof.

In order to best understand our invention, reference should first be had to the experimental procedures which We employed.

Experimental procedures In our experimental work, a series of alloys were melted under an inert atmosphere in a nonconsumable electrode arc furnace using a water-cooled, copper hearth. High purity materials (greater than 99.5%) were used for the alloy charges and generally weighed about grams. These procedures are, of course, quite well known to those skilled in the art.

The alloy samples as thus prepared were subsequently nitrided. For nitriding, we used a cold wall furnace employing a molybdenum heating element and radiation shields with the furnace being evacuated to five microns pressure and flushed with nitrogen prior to heating. Temperatures were measured with an optical pyrometer, namely, a Leeds and Northrop optical pyrometer, catalog number 862, sighting on an unnitrided molybdenum heating element which completely surrounded the specimens. The temperatures given herein are corrected from this source. We used a correction factor determined by using a tungsten-rhenium thermocouple in conjunction with the sightings of the aforesaid optical pyrometer.

The nitrided structures, thickness and microhardness of the various reaction zones or layers were determined using standard metallographic techniques. The surface hardness was also measured with a Rockwell hardness tester using a conical diamond indentor at a load of 60 kg. [Rockwell- A Scale (Ra)].

Erosion resistance was determined by using a tester that impinged high velocity particles on the specimen surface at a angle. In this erosion tester, an adaptation of the S. S. White Airbrasive unit, abrasive particles sifting through holes in the floor of a vibrating storage chamber are swept from the lower chamber by a stream of argon gas, under controlled pressure, towards a nozzle (0.018" 'I.D.) which directs the particles in a jet against the surface of the test specimen. The specimen and nozzle are mounted in a fixture encased in a transparent Plexiglas chamber. The mounting fixture provides means for accurately adjusting the nozzle-to-coating distance and erosion angle. Spent abrasive is removed from the test chamber by an ordinary vacuum cleaner connected to one side of the chamber. The erosion test is timed with a stop watch, and

the depth of the erosion attack is measured with a-bench' mounted dial-indicator gauge.

The propellant gas used in the erosion tester is argon. The abrasive powder, S. S. White Airbrasive Powder No. 1 is described by the manufacturer as a fine crystalline grade of aluminum oxide with an average particle size of 2.7 microns, and containing only a small proportion of particles smaller than 10 or larger than 5311. (270 mesh).

The S. S. White Industrial Airbrasive unit, Model F, used in the erosion testing required several modifications to insure steady and repeatable low abrasive flow rates (-05 gm. per minute). These changes included alteration of the orifice plate in the mixing chamber, and additions to the electrical control circuit. The abrasive flow (quantity of abrasive leaving nozzle per unit time) is controlled by regulating the voltage applied to the mixing chamber vibrator with a Powerstat and a rheostat in series in the control circuit.

The standard nozzle-to-specimen erosion distance was set at 0.400. Determination of the vertical distance between the nozzle and specimen for a given angle and erosion distance requires consideration of the nonle diameter.

7 Erosion test results are reported as erosivity numbers in seconds-per-mil of sample removed, at the standard abrasive flow rate and gas pressure. They are calculated by dividing the erosion time in seconds by the erosion pit depth inrnils. All measurements are made with a bench mounted dial-indicator gauge calibrated in 0.0001". A sharp 60 included angle point is used to measure the deepest area in the erosion pit and referenced to the unabraded area surrounding the erosion spot. The use of a dial-indicator gauge is considered superior to the use of pointed micrometers due to its constant gauging pressure and lower dependence on operator technique.

DESCRIPTION OF THE INVENTION We have discovered a novel group of erosion resistant materials characterized by a hard, continuous surface layer which is supported by an underlying hard zone consisting of relatively fine dispersed phases in which the hardness grades inwardly from the surface layer. Such materials are formed when alloys within our prescribed compositional ranges as hereinafter taught are reacted with nitrogen or an environment which is nitriding to the alloys at elevated temperatures. I

Most of the unreacted alloys included within the scope of our invention are single phase solid solutions over the range of temperatures employed for nitriding. As the nitriding reaction proceeds, the nitrogen diffuses inwardly resulting in formation of a continuous nitride layer on the surface beneath which there is developed a multiphase structure-that is, a microstructure consisting of two or more phases, usually differing in nitrogen content as well as metallic content, which are discernible when observed in cross section under a microscope using typical metallographic techniques. We would note, to avoid any misunderstanding, that the term phase as used herein means a physically homogeneous and distinct portion of a materials system and that multiphase means two or more of such phases. Of course, some hardening can occur even below the metallographically observed reaction zone by the presence of nitrogen in solid solution.

The titanium or zirconium present in the alloy reacts most readily with nitrogen to form nitrides. Tantalum, columbium and vanadium will, of course, also form nitrides. Tungsten and molybdenum, which do not form stable nitrides at these temperatures, exert a very beneficial influence on the erosion resistance of our nitrided materials. The presence of these elements appears to promote the formation of a finer nitride dispersion resulting in greater hardening.

The influence of these alloying elements on the hardness and grading of the resulting nitrided material can be seen by comparing the following microhardness data:

Microhardness (DPN) at the Both materials were nitrided at 3250" F. for two hours. The thickness of the continuous nitride surface layer was 1.5 'rnils' for the'unalloyed columbium and 2.5 mils for the Cb-7Ti-5W alloy. The sharp discontinuity in hardness betweenthe continuous nitride layer andthe underlying material in unalloyed columbium is evident. Conversely, the rather smooth transition in hardness '(gradingLand greater hardening of the nitrided alloy is readily apparent. This same effect may alsobe seen by comparing Rockwell-A (Ra) hardness measurements made directly on the surface. These values are Ra 59 for nitrided unalloyed columbium and Ra 79 for the nitrided alloy. The difference in erosion resistance for these two materials may be seen from the data presented in Table I.

TABLE I.EROSION DATA FOR NITRIDED MATERIALS The improved erosion behavior of appropriately alloyed nitrided materials based on tantalum and vanadium may also be seen from the foregoing table. In general, it is desirable to nitride the vanadium-base alloys at even lower temperatures to minimize the development of porosity in the surface layer.

For most applications the desired weight pickup during nitridingis greater than 1 mg./cm. However, for some applications, particularly when the alloys are used in very thin sections or as a coating on another material, the amount of nitriding may be from 0.1 to 1 mg./cm.

These nitrided alloys also have a greater volume of dispersed phase beneath the continuous nitride layer and show improved grading compared to the nitrided elemental materials. This same desirable efiect is noted where zirconium is present in the alloys rather than titanium. We have evaluated the alloys Cb-5Zr-5W and Cb-SZr- 5M0. Zirconium and titanium may be combined in these dilute alloys to produce the desirable effects noted herein. Similarly, tungsten and molybdenum may be combined and elements of the group Ta, Cb, V may be combined and interchanged.

In considering the composition limits that define the nitrided alloys of our invention, it is necessary to consider the following groupings:

Group I is tantalum, columbium and vanadium Group II is titanium and zirconium Group III istungsten and molybdenum.

elements from When vanadium alone is present of the Group I, the minimum vanadium content is 90 percent and the maximum of either Group II or III content is 8 percent.

When one or more elements of Group I is present, the minimum Group I content and the maximum of either Group II or III content is determined by a linear proportional relationship involving the weight fractions of the individual Group I elements present in the alloy. These weight fractions are as follows:

Thus, when one or more elements of Group I are present, the minimum Group I content is given by Min. Group I=88A+85B+90C and the maximum of either Group H or III is given by Max. Group II or III=10A+13B+8C These alloys may be reacted in our nitriding environments like argon-2.5 percent nitrogen with similar weight pickup and hardening. Thus, a variety of nitrogen containing environments can be used to produce similar hardened materials. However, upon reacting in argon-0.1 percent nitrogen under low gas flow conditions, the effect of lowered nitrogen availability is observed and a somewhat modified reaction product is obtained. Since our surface reacted composites are in a thermodynamically metastable condition, those skilled in the art will realize that a variety of heat treatments, including multiple and sequential treatments, can be used to modify the reaction structure and resulting properties whether performed as part the overall nitriding reaction or as separate treatments. We have also nitrided at higher temperatures (and times) that normally would produce some embrittlement and then subsequently annealed in inert gas as a tempering or drawing operation to improve toughness. This duplex treatment results in a deeper reaction product with the hardness-toughness relationship controlled by the tempering temperature and time.

The present useful alloys could be produced by powderprocessing techniques. Also, such alloys could be employed on another metal or alloy as a surface coating or cladding and with the proper selection, a highly ductile or essentially unreacted substrate can be obtained. For example, columbium or tantalum are much less reactive to nitrogen when used in conjunction with the alloys and molybdenum is inert to nitrogen. The nitrided material can be used as a mechanically locked insert or it can be bonded or joined by brazing, for example, to a substrate.

We have also observed the excellent corrosion resistance of both the alloys and the nitrided alloys in strong acids, and these materials could effectively be employed for applications requiring both corrosion and abrasion resistance. Both the alloys and the nitrided alloys possess good structural strength. Thus, the materials can be employed for applications involving wear resistance and structural properties (hardness, strength, stiffness, toughness) at room and elevated temperatures. Other useful properties of the nitrided materials include good electrical and thermal conductivity, high melting temperature, and thermal shock resistance.

Although the alloys receptive to nitriding can be pro duced by coating or surface alloying techniques, many uses involve the forming and machining of a homogeneous alloy. One of the advantages in utility of these materials is our ability to form the metallic alloys by cold or hot working and/or to machine (or hone) to shape in the relatively soft condition prior to final nitriding. Only minimal distortion occurs during nitriding and replication of the starting shape and surface finish is excellent. The final surface is reproducible and is controlled by original surface condition, alloy composition, and nitriding treatment. For some applications, the utility would be enhanced by lapping, polishing, or other finishing operations after nitriding. The nitrided surface is quite hard but only a small amount of material removal is required to produce a highly finished surface.

The excellent erosion and wear resistance of the nitrided materials can be effectively employed with the other useful properties of the alloys and nitrided materials to produce a wide range of products. Some of these are: dies for extrusion, drawing, and other forming operations, armor, gun barrel liners, 'EDM (Electrical Discharge Machining) electrodes, spinnerets, guides (thread, wire, and other), knives, razor blades, scrapers, forming rolls, grinding media, capstans, needles, gages (thread, plug, and ring), bearings and bushings, nozzles, cylinder liners, pump parts, mechanical seals such as rotary seals and valve components, engine components, brake plates, screens, specialized electrical contacts, fluid protection tubes, crucibles, molds and casting dies, and a variety of parts used in corrosion-abrasion environments in the papermaking or petrochemical industries, for example.

It will be understood that various modifications and variations may be affected Without departing from the spirit or scope of the novel concepts of our invention.

We claim as our invention:

1. A nitrided ternary or higher alloyed material consisting essentially of:

,(a) at least one metal of the group consisting of columbium, tantalum and vanadium;

(b) at least one metal of the group titanium and zirconium;

(c) at least one metal of the group molybdenum and tungsten; and wherein:

(d) the minimum content of the group titanium and zirconium is 2 percent;

(e) the minimum content of the group molybdenum and tungsten is 2 percent;

l(f) the minimum content of the group columbium,

tantalum and vanadium is determined by the formula (g) the maximum content of the group titanium and zirconium is determined by the formula (h) the maximum content of the group molybdenum and tungsten is determined by the formula 10A+ 13B (i) and wherein in the foregoing (j) which nitrided material is characterized by a continuous nitride layer on the surface thereof below which there is a graded multiphase nitride structure lessening in nitride content from the surface layer inwardly; and

(k) wherein the nitrogen weight pickup is at least 0. 1

milligram per square centimeter. 

